1. Field of the Invention
The present invention relates to a signal reproducing apparatus for reproducing an information signal recorded on a magnetic recording medium and, more particularly, to a signal reproducing apparatus using a reproducing method by the displacement of a magnetic wall.
2. Related Background Art
Hitherto, as an apparatus for reproducing an information signal recorded on a magnetic recording medium, various apparatuses have been known. Among them, a magneto-optical recording medium, a reproducing apparatus, and a reproducing method proposed in JP-A-6-290496 are effective means for remarkably raising a recording density of an information signal because micro information signal marks can be reproduced while exceeding a diffraction limit of light that is used for reproduction.
FIGS. 1A and 1B are diagrams showing a construction of a magneto-optical recording medium 1 which is used in the above prior art. FIG. 1A is a plan view and FIG. 1B is a cross-sectional view. In FIGS. 1A and 1B, the magneto-optical recording medium 1 comprises a transparent substrate 40 and a magnetic layer 41 formed on the substrate 40. The magnetic layer 41 is constructed by laminating a first magnetic layer (magnetic wall displacing layer) 42, a second magnetic layer (switching layer) 43, and a third magnetic layer (magnetic recording layer) 44. Signal tracks 45 on which an information signal is recorded are formed on the magnetic layer 41. In at least the magnetic wall displacing layer 42, the adjacent signal tracks 45 are magnetically separated. The magnetic wall displacing layer 42 comprises a perpendicular magnetization film in which a magnetic wall coercivity is smaller and a magnetic wall mobility is larger than those of the magnetic recording layer 44. The switching layer 43 comprises a magnetic layer whose Curie temperature is lower than those of the magnetic wall displacing layer 42 and magnetic recording layer 44. The magnetic recording layer 44 comprises a perpendicular magnetization film.
In FIGS. 1A and 1B, information signal marks serving as upward and downward perpendicular magnetization regions are recorded in the magnetic recording layer 44. A magnetization in the magnetic recording layer 44 is also transferred to the magnetic wall displacing layer 42 through the switching layer 43 by an exchange-coupling force acting between the magnetic layers at room temperature. Each of the upward and downward arrows in the diagram indicates a direction of the magnetization. In each layer, magnetic walls Q1, Q2, . . . , and Q9 are formed among the information signal marks magnetized in one direction and the information signal marks magnetized in the reverse direction existing before and after the above information signal marks, respectively.
A principle of the signal reproduction in the above prior art will now be described. In the case of reproducing an information signal, while moving the magneto-optical recording medium 1, a heating light beam for heating the magnetic layer 41 of the magneto-optical recording medium 1 and a reproducing light beam for detecting a state of magnetization as a signal by a magneto-optical effect are irradiated from an optical head to the magnetic layer 41 of the magneto-optical recording medium 1. Although those light beams also can be separately provided, an example in which a light beam only for heating is not provided but a function to heat the magnetic layer 41 is also provided for the reproducing light beam is also disclosed in JP-A-6-290496. With such a construction, there are advantages such that the size and weight of the optical head can be reduced and low costs can be realized because of reasons such that it is sufficient to use one light source, there is no need to perform a relative positional adjustment of the heating light beam and the reproducing light beam, and the like. An example of such a construction will now be described.
FIGS. 2A and 2B are diagrams for explaining the principle of the signal reproduction. FIG. 2A is a plan view and FIG. 2B is a cross-sectional view. In the diagram, reference numeral 46 denotes a light beam irradiated by the optical head. The light beam 46 is irradiated from the magnetic wall displacing layer 42 side so as to converge a micro light spot 47 to the magnetic layer 41 of the magneto-optical recording medium 1. The light beam 46 is relatively moving in the direction shown by an arrow A for the magneto-optical recording medium 1. When the light beam 46 is irradiated as mentioned above, the magnetic layer 41 is heated and a temperature distribution is generated having a peak at a position P that is deviated backward relative to the center of the light spot 47 in its moving direction. Reference numeral 48 denotes an isothermal line indicative of a region where the temperature reaches Ts as a temperature near the Curie temperature of the switching layer 43. The temperature of the magnetic layer 41 rises while exceeding the temperature Ts at a position Xs that is deviated to the front side of the light spot 47. After the temperature reaches the peak at the position P, it starts to decrease and is lower than the temperature Ts at a position Ys that is deviated to the back of the light spot 47.
At a position away from the heating portion by the light beam 46, the temperature of the magnetic layer 41 is sufficiently low, the magnetic wall displacing layer 42 is exchange-coupled to the magnetic recording layer 44 through the switching layer 43, and the temperature distribution of the magnetic layer 41 is almost uniform. Therefore, a force such as to displace the magnetic walls transferred to the magnetic wall displacing layer 42 does not act, so that the magnetic walls are fixed. A temperature of the portion which has reached the position Xs of the switching layer 43 rises to Ts and the magnetization disappears. Therefore, the magnetic wall (magnetic wall Q5 in the example shown in the diagram) which has reached the position Xs is not restricted by the exchange-coupling force in the magnetic wall displacing layer 42 but is subjected to a force by a gradient of the temperature. Thus, in the magnetic wall displacing layer 42, the magnetic wall Q5 is displaced in the direction shown by an arrow B in which the temperature is higher and a magnetic wall energy is low, namely, toward the peak position P of the temperature. Therefore, at the irradiating position of the light spot 47 of the magnetic wall displacing layer 42, as shown in the diagram, an enlarged magnetization region of a predetermined size is formed irrespective of a size of an original information signal mark (magnetization region). By using a magneto-optic effect caused by that magnetization region, a change in a signal corresponding to the displacement of the magnetic wall as mentioned above is detected by a reflected light of the light beam 46.
The magnetic walls Q1, Q2, . . . , and Q9 which were transferred to the magnetic wall displacing layer 42 as mentioned above sequentially move toward the peak position P of the temperature each time they reach the position Xs in association with the displacement of the light beam 46. A change in a signal corresponding to such a displacement is detected by the optical head. A displacing speed of the magnetic wall is sufficiently higher than a speed of the relative displacement of the light beam 46. Therefore, the signal change is very fast. Even when a length of an information signal mark is smaller than a diameter of light spot 47, since the signal is detected from the enlarged magnetization region of a predetermined size, an amplitude of the signal change is not reduced. A detection signal in which an amplitude is constant and a waveform is close to a rectangular wave can be obtained irrespective of the length of the information signal mark.
In the above prior art, in a case of locally heating the magneto-optical recording medium and displacing the magnetic wall, as mentioned above, in addition to the displacement of the magnetic wall directing from the forward of the peak position of the temperature toward the peak position of the temperature (this displacement is now referred to as a first magnetic wall displacement), a magnetic wall displacement directing from the back of the peak position of the temperature toward the peak position of the temperature (this displacement is now referred to as a second magnetic wall displacement) also occurs. The first and second magnetic wall displacements will now be described with reference to FIGS. 3A and 3B.
FIGS. 3A and 3B show states in which the time elapses from the state shown in FIGS. 2A and 2B, the light beam 46 moves, and the magnetic wall Q5 of the magnetic wall displacing layer 42 reaches the position Ys that is deviated backward of the light spot 47. At this position, the temperature of the magnetic layer 41 is lower than Ts and the magnetization again appears in the switching layer 43. Thus, the magnetization of the magnetic recording layer 44 is transferred to the magnetic wall displacing layer 42 through the switching layer 43, so that a micro magnetization region is again transferred to the magnetic wall displacing layer 42 together with the magnetic wall Q5.
The magnetic wall Q5 is subjected to a force due to the gradient of the temperature and displaces in the direction shown by an arrow C in which the temperature is higher and the magnetic wall energy is lower, namely, toward the peak position P of the temperature. (Strictly speaking, the magnetic wall Q5 itself is not displaced but a magnetic wall Q5' which appears on the front side of the micro magnetization region to constitute a pair together with the magnetic wall Q5 displaces. However, such a displacement is expressed as a second magnetic wall displacement of the magnetic wall Q5 for convenience of explanation.) That is, the magnetic wall causes the first magnetic wall displacement as mentioned above at a time point when it reaches the position Xs. The second magnetic wall displacement occurs at a time point when the time further elapses and the wall reaches the position Ys.
In the construction also providing the function to heat the magnetic layer 41 for the reproducing light beam without using the light beam only for heating, the peak position P of the temperature generally occurs on the inside of the light spot 47 in the temperature distribution which is formed by the irradiation of the light beam 46. Therefore, not only the signal change by the first magnetic wall displacement but also the signal change by the second magnetic wall displacement are included in the signal that is detected by the optical head. This point will be described further in detail with reference to FIGS. 4, 5A, 5B, 5C and 6.
FIG. 4 shows an example of a recording state of information signal marks of the magnetic recording layer. The information signal which is recorded here is expressed by 0 and 1 and the signal is recorded by using what is called a mark edge recording method such that boundary portions between the information signal mark and the information signal marks before and after it, namely, the magnetic wall is made to correspond to 1 and the portions other than the magnetic wall are made to correspond to 0. A time duration of the information signal mark to be recorded is equal to nT (n is an integer of 1 or more and T is a clock period).
In this instance, ideally, the detection signal of the optical head ought to have a signal waveform in which the level changes in correspondence to the first magnetic wall displacement, as shown in FIG. 5A. Actually, however, as shown in FIG. 5B, a signal waveform including a level change corresponding to the second magnetic wall displacement, namely, a signal which is delayed from a signal of FIG. 5A by only a time Td that is required until the magnetic wall displaces from the position Xs shown in FIG. 2A to the position Ys is superimposed to the ideal signal of FIG. 5A, so that a signal waveform as shown in FIG. 5C is derived.
However, for example, if the operator intends to detect a change in signal level by using means for performing a comparison with a predetermined slice level Vs or the like from such a signal as is usually executed and to obtain a pulse signal corresponding to 1 of the information signal as shown in FIG. 6, the signal cannot be correctly reproduced. In other words, when there is a slight level fluctuation as shown by a broken line in the signal of FIG. 5C, a situation such that erroneous pulse signals are generated as shown in FIG. 6 or, contrarily, pulse signals 52 and 53 to be inherently detected are dropped out occurs. There is, consequently, a problem such that the correct information signal cannot be reproduced.
If means such that the slice level is divided into a plurality of levels so that the change in signal level corresponding to the first magnetic wall displacement can certainly be detected even if a fluctuation in signal level occurs is used, on the contrary, it is likely to erroneously detect the change in signal level corresponding to the second magnetic wall displacement, so that the improving effect cannot be derived. Further, a time difference Td between the timing of the occurrence of the first magnetic wall displacement and the timing of the occurrence of the second magnetic wall displacement is not constant, but variable, and fluctuates depending on a magnitude of the light beam, an environmental temperature, and the like. The signal which is reproduced in association with such a fluctuation also causes a time fluctuation and becomes a cause of erroneous reproduction of the signal. This point will now be described with reference to FIGS. 7A to 7C.
FIG. 7A shows an ideal signal waveform corresponding to the first magnetic wall displacement. FIG. 7B shows a signal waveform which is delayed by only a time Td1 and corresponds to the second magnetic wall displacement. FIG. 7C shows a signal waveform in which the signals of FIGS. 7A and 7B are superimposed. FIG. 7C shows a state in which a signal change q111 in one direction of the signal waveform corresponding to the first magnetic wall displacement of a magnetic wall Q11 in FIG. 7A and a signal change q102 in the reverse direction in the signal waveform corresponding to the second magnetic wall displacement of a magnetic wall Q10 in FIG. 7B simultaneously occur due to the time fluctuation of the reproduction signal as mentioned above. Particularly, in the case where a time duration of the information signal mark constituting the information signal is equal to nT (where n is an integer of 1 or more and T is a clock period), such a state occurs when the time difference Td1 between the timing of the occurrence of the first magnetic wall displacement of a certain magnetic wall and the timing of the occurrence of the second magnetic wall displacement of the same magnetic wall is equal to Td1=mT (where m is an integer of 1 or more). For example, FIG. 7C shows a case where Td1=mT.
However, when the signal changes q111 and q102 simultaneously occur as mentioned above, as shown in FIG. 7C, the change in signal level corresponding to the first magnetic wall displacement of the magnetic wall Q11 is cancelled by the change in the reverse direction of the signal level corresponding to the second magnetic wall displacement of the magnetic wall Q10. Therefore, the change in signal level corresponding to the first magnetic wall displacement is small. Even if it is compared with the slice level Vs or even if any other detecting means is used, it is difficult to certainly detect the signal change. Thus, if a pulse signal is tried to be obtained by detecting the signal change corresponding to 1 of the information signal, pulse signals 54 and 55 to be inherently detected are dropped out as shown in FIG. 8. There is also a problem such that the correct information signal cannot be reproduced.